Life Cycle - Natural Environment (4B)

  • Classifying organisms that live in the San Francisco Bay.
  • Creating a San Francisco Bay mud fauna food web. 
  • food web
  • food chain
  • primary consumer
  • primary producer
  • secondary consumer
  • Life Cycle - Natural Environment (4B) 
  • Magnifying lens
  • Swift-GH Microscope 
  • Murder in the Mud by M. Doherty and J. Blueford

Students look at organisms from the San Francisco Bay mud. 


The life of the San Francisco Bay is dominated by little critters.  None of the organisms of the  San Francisco Bay mud life are exceptionally beautiful nor unique, but all are part of a  food web. The picture to the left was taken at Mud Slough which flows into the southern part of the San Francisco Bay in the City of Fremont.  

In an ecosystem, organisms play different roles in the food web.  There are primary producers, primary consumers, and then different levels of secondary consumers.  

However before you determine the position of an organisms in a food web you need to identify the organisms first.   In this lab, the students will first take a look at the different organisms  of the mud  (from the San Francisco Bay) and then they will analyze their positions in the food chain.

This lab can be customized to either a lake, pond, river, or marsh in your area.  However, bays or along a coastline are preferable.   Look for a place where the currents may concentrate shells.  In lakes or ponds you may look at where some birds eat and find left over shells. 

  1. Discuss with students how they can determine the role that an organism plays in an ecosystem. Review that the plants are primary producers (produce their own food through photosynthesis)  and the smaller animals like snail are primary consumers (first eat primary producers).  A secondary consumer would be an animal that eats  primary consumers.  The food chain can have several layers, and several levels can be eaten by more than one organism.  
  2. Read "Murder in the Mud"  to students.  The story is just a fun way to point out which organisms eat one another.  The carnivore, or Hannibal Radula, contains a tooth like structure called a radula that can actually bore through the shell of another organism that will be its next dinner.   





  3. Biologists classify organisms because it is easy to talk with other biologists if the name is standardized.  When students select the different groups, have them name the organism as a class.  The scientific name is fine, but naming their own organism can even be fun.  Try to get them to name the organism to reflect what that shell looks like.  Have them select two parts of the name (Genus + species).   You can use the analog that people's names help identify them, so genus + species helps scientists identify the different organisms.

    Have the students draw and describe the samples of sea life by name and key characteristics.  The students need not name the sea life by their scientific names.  They can devise or invent names for the organisms.  However, if you want them to use the correct scientific names which is described below. 
  4. In your kit you have 5 to 7 different types of organisms on the species level.  You may have one or two organisms that are not in the following description.  These mollusks represent only a fraction of the population.  These shells were collected under the Dumbarton Bridge (between the cities of Fremont and Menlo Park, California).  The proportion that is in your container reflects the ratio in which they were found, which changes seasonally.  You can discuss with your students why some are more abundant than others.  This has to do with which organisms are more abundant but also how they were transported after they died.

  5. Using these specimens you can devise several activities with your students, not only the prescribed lab.  Mix all the shells together and have them separate the different groups.  Depending on the verbal and written skills of your students, you may want them to start writing a description of the little organisms.  Direct their attention to size, shape, color, and ornamentation on the shell.  You can also have them draw the different types, as drawing tends to really develop their observational skills.


On the information below, the position of each organism in the food chain is outlined, and a food web can be created.


Gemma gemma - Shell is no longer than high, slightly triangular; general color white with purple tinge, shell very thin, hinge and teeth very reduced.  Introduced to the San Francisco Bay from the east coast.  This bivalve is a filter feeder, meaning that it takes in water and "filters" the algae from it to digest.  It is considered a primary consumer.
Macoma nasuta - Also called the bent-nosed macoma.  Lives in shallow water with muddy bottoms.  Some are 3-4 cm long, color grayish white.  Anterior end broadly rounded; posterior end bluntly pointed, partially truncated and noticeably bent to one side and is a filter feed like G. gemma.
Iscadium demissum - Also known as the ribbed horse mussel has fine ribbed lines running lengthwise. This purple mussel grows to 3.5 inches. Pearly luster inside of shell.
Mytilus edulis - Shell elongated -triangular, rather  plump, with scarcely noticeable beaks at the apex.  Length about 3 inches on the average.  Adult shells are deep bluish black with a shiny periostracum (outer covering); juveniles show various shades of gray, green and brown, often exhibiting rays of color.  Also called the bay mussel, lives in rock areas in colonies.  M. edulis is a filter feeder with very few natural enemies.

Ostrea lurida - No longer than 3-5 cm long, the shape quite irregular, depending on the surface of the object on which it grows.  Shells are not especially thick or heavy.  Lives in shallow waters with stony bottoms.  This is a common native oyster along the west coast and is also a filter feeder.


Ocenebra interfossa - A spindle-shaped shell about 3/4 of an inch high.  There are 5 whorls, high spiral, and a sharp apex.  Lives in the shoreline or on rocks.  Commonly called the sculptured rock shell, because of the large wavy ridges that revolve with the whorls.  This little gastropod is top of the food chain in the mud, eating many of the bivalves and other gastropods.

Nassarius obsoletus - Black Dog Whelk; medium sized, surface blackish with obscure spiral and longitudinal lines, introduced and extremely abundant in the San Francisco Bay (mud snail), nearly one inch high, with about 6 whorls.  Apex rather blunt, and commonly more or less eroded.  The only sculpture consists of weak revolving lines, plus a few vertical folds on the early whorls.  Inner lip deeply arched.  Color deep purplish black.  This is an east coast snail, probably introduced into California water with young oysters.  It is a scavenger that eats dead fish or other organic debris. 

Nassarius tegulus - Commonly called the Mud Dog Whelk.  Lives in the mudflats, about 3/4 of an inch high, a stocky shell of 5 or 6 whorls with a sharply pointed apex.  Sculpture of weak revolving lines, sometimes faintly banded.  Inner lip broadly expanded, outer lip thickened.  This group eats similar to N. obsoletus and is hard to distinguish between the two.

Turritella sp.  - Greatly elongated, many whorled shells.  A large group of organisms that live mainly in tropical waters.  A few will venture into he San Francisco Bay, but this group is very rare.  This groups eats food similar to that eaten by N. obsoletus.

Acmaea sp - Limpet is the common name for this group.  Shells conical, oval, and open at base, with no opening at the top.  No spiral at any stage of growth and does not have the pearly look to its inside shell.  These gastropods live on stones and grasses at the shoreline, generally between the tide limits.  They are herbaceous which means they eat small algae or bacteria.

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